Re: Can't get custom allocation function to work

["Martin C. Martin" <martin@...>]

Ralph Hempel wrote:
> kathrin_69@gmx.de wrote:
> > Hi,
> >
> > I'm trying to replace the Lua default l_alloc function with my own 
> > one. My aim is to have all memory Lua allocates inside a single array 
> > of pre-allocated memory.
>
> <snip>
>
> > void* alloc_impl_manual( void* ptr, size_t osize, size_t nsize )
> > {
> >   if (nsize)
> >   {
> >       std::vector<unsigned char>* mem = new std::vector<unsigned 
> > char>(nsize);
> >                //yes, it will leak. don't care for now
> >
> >       return &(*mem)[0]; //returned memory position should be aligned 
> > by new
> >   }
> >   return NULL;
> > }
>
> Ummm, besides the leak, should you not be returning:
>
>   return( mem )
>
> since mem is already a pointer to the memory you allocated?

mem is a pointer to a "header", usually something like this:

class vector<T> {
  T *storage;
  size_t size; // Amount actually used
  size_t allocated; // number of elements in storage, >= size.
}

So, if you return mem, you're returning the address of that "header", 
not the array you've allocated.

> And this expression  &(*mem)[0] is very strange. I parse that as:
>
> (the adress of ( (the contents of mem) ???? )
>
> my compiler warns that the subscripted value is neither an array nor a
> pointer...

vector overloads [] so that (*mem)[0] equals storage[0], which is 
exactly what you want the address of.

Best,
Martin